1. Field of the Invention
The present invention relates to a magnetic velocity selector for passively selecting slow neutral particles, such as atoms, molecules, or neutrons, from a source having both slow and fast neutral particles. In particular, the present invention uses a magnetic field placed along a curve as a low-pass or band-pass velocity filter to provide a source of slow or monoenergetic neutral particles for such purposes as loading traps, producing a beam of coherent particles (e.g. xe2x80x9catom laserxe2x80x9d), directly depositing particles onto surfaces, and for applications such as time and frequency standards (e.g. atomic clocks), spectroscopy, particle-surface scattering measurements, particle interferometers, crystallography, and particle scattering studies.
2. Description of Related Art
Given the widespread importance of slow or monoenergetic neutral particle sources, much effort has been dedicated to improving their performance and especially to their simplification. Mechanical velocity selection of particle beams has been a standard method for producing narrow velocity distributions, particularly for crossed molecular beam and neutron beam scattering experiments [1]. Methods employed for mechanical velocity selection typically use the motion of some assembly to physically block particles moving outside of a specified range of velocities. Examples of these assemblies include sets of rotating slotted disks, spinning grooved plates or cylinders, and rotating helical fins. Transmitted particles move through the moving assembly without physically contacting it, while blocked particles reflect or stick to the assembly surfaces that move across their path. The main disadvantages of these methods are that these devices (1) are extremely complex, (2) require rapidly moving parts inside of the vacuum, and (3) produce small total efficiencies for slow atoms. The present invention is much simpler, more economical, and more efficient, and more compatible with vacuum technology
Atom traps, which are widely used for both scientific and technological applications, are loaded from sources of slow atoms. Lasers have been used to actively slow the fast atoms in a beam, thereby compressing the velocity distribution and increasing the flux of slow atoms. Laser methods require no mechanical components to be placed in the vacuum region. Trap loading methods that rely on laser slowing an atomic beam, therefore, can achieve high load rates and low background pressures, resulting in long trap lifetimes. Zeeman slowing [2] is the most successful of these techniques, providing typical load rates of 108 atoms/s into a magneto-optical trap (MOT). Under optimum conditions, rates as high as 1011 atoms/s have been attained [3]. Unfortunately, this method is complex and expensive since it usually requires acousto-optical and/or electro-optical modulators, and significant laser power. Additionally, the slowed atomic beam expands transversely as it propagates away from the beam source, leading to a decrease in the beam intensity. Finally, laser slowing methods are not useful for molecular beams because the internal structure of molecules is much more complicated than for atoms. In contrast to laser slowing, the present invention is not comprised of lasers or other optical devices and the invention can be used to produce a beam of slow or monoenergetic molecules or neutrons.
MOTs have also been directly loaded from the slow atoms present in a vapor cell [4]. The main advantage of vapor loading lies in its simplicity since no additional laser beams, other than those used for trapping, are required. Load rates as high as 1011 atoms/s have been achieved in vapor cells [5], though the relatively high background gas pressure results in reduced trap lifetimes that prove unsuitable for many applications. This limitation encouraged the development of the double-MOT scheme. In this technique, the trapped atoms from a vapor-loaded MOT are transferred to an ultra-high vacuum (UHV) chamber using magnetic guiding, providing load rates of xcx9c108 atoms/s [6]. The main disadvantage of this method, as for Zeeman slowing, is the degree of complexity and expense. MOTs have also been loaded directly from a thermal atomic beam [7], achieving load rates of xcx9c107 atoms/s from an oven located xcx9c20 cm from the trap [8]. Although simple, this method suffers from reduced trap lifetimes resulting from the proximity of the relatively high-pressure atomic source. Applications that require less than maximal particle flux, but must be UHV-compatible, may benefit from a simple technique that does not involve lasers, such as the one described here.
Myatt et al. [6] previously used magnetic fields to guide already slow atoms from one MOT to another. Meschede et al. [9] and Goepfert et al. [10] used a combination of light forces and permanent magnets to deflect the laser-slowed atoms out of an atomic beam. In their work, a Zeeman-slowing system is used to create slow atoms and a transverse laser beam optically deflects the atomic beam. The major conceptual improvement over the work of Myatt, Meschede, and Goepfert which the present invention provides, is the complete lack of laser manipulation of the atomic beam. In the present invention, neutral particles are passively selected according to their velocity. Furthermore, since the neutral particles are passively selected, the particles are in the ground state and there is no spontaneous emission as occurs with laser slowing. Therefore, the quantum mechanical state of the particle is preserved. The present invention provides an exceptionally simple, economical and robust alternative to laser cooling methods.
According to the invention, neutral particles, such as atoms and molecules, are selectively conveyed or filtered according to each particle""s velocity as the neutral particles are transported along a path having at least one curved region by generating an inhomogeneous magnetic field across a cross-section of the path. The neutral particles may be transported through a physical tube or simply through the region defined by the magnetic field. The path may have more than one curved region and may additionally have one or more straight regions. In one embodiment of the invention, the magnetic field is generated by homogeneously or inhomogeneously magnetized permanent magnets. In another embodiment of the invention, the magnetic field is generated by wires. In another embodiment of the invention, the magnetic field is generated by current conducting elements which have been deposited on a surface using lithographic and/or deposition techniques. Additionally, magnetic materials or yokes may be used in order to focus and contain the magnetic field lines.